The present invention relates to a method and system for recording and verifying three-dimensional radiation dose distributions using an anthropomorphic film phantom.
Stereotactic radiosurgery is a method for treating brain lesions, using collimated convergent beams of x-ray photons produced by a clinical linear accelerator. In order to conform the administered dose distribution to the delineated volume of the lesion, while sparing healthy adjacent tissue, the method requires an extremely high spatial accuracy of approximately xc2x11 millimeter (mm). The method also requires an accuracy of xc2x12% in controlling the magnitude of the administered dose.
Because the success of stereotactic radiosurgery hinges on the accurate delivery of dosage of x-ray photons to the lesion, simulated radiosurgery using a suitable phantom, or a pseudo-object, is performed prior to actual application of the radiosurgery to a human patient, to record and verify the resulting dose distribution. The result of the simulated radiosurgery may be used to adjust stereotactic radiosurgery parameters to ensure that the desired dose distribution is applied to a human patient. Currently, ionization chambers, diodes, and diamond detectors are used to measure radiation dose distribution. (See, for example, U.S. Pat. No. 5,635,709.) Unfortunately, such devices provide a dose measurement at a single point at a time. Alternatively, radiosensitive polymer gels have been used as prototypical three-dimensional dosimeters as described, for example, in U.S. Pat. No. 5,633,584. Unfortunately, such gels are not well established, and require an expensive magnetic resonance imaging (MRI) scanner in order to read the recorded dose distribution.
The present invention provides a method and system for recording and verifying a dose of radiation to be administered to a subject in three dimensions simultaneously, using an anthropomorphic Film phantom. The method and system offer high spatial resolution, and the capacity to integrate over time the dose produced by multiple beams.
The phantom of the invention includes a hollow shell in the form of, for example, a human head, which is made of tissue-equivalent material and filled also with tissue-equivalent material. The phantom also includes a box made of tissue-equivalent material, which is adapted to be adjustably securable within the hollow shell. The phantom further includes multiple layers of film separated by multiple spacers, which are replaceably loaded into the box.
According to the method of the present invention, first, the hollow shell is provided. Mounted in the hollow shell is the box. Next, an image of a lesion to be treated is registered into treatment-planning software. For this purpose, an image or three-dimensional contour of an actual lesion may be transferred to the software. Alternatively, a simulated lesion modeled after an actual lesion may be formed and secured in the box, which is adjustably positioned and secured within the hollow shell, and an image may be taken of the hollow shell including the box that contains the simulated lesion. To take an image of the simulated lesion, a computed tomography (CT) or magnetic resonance imaging (MRI) scanner may be used. The image of the simulated lesion is then transferred to the treatment-planning software, and the simulated lesion is removed from the box. Next, the image of a lesion thus registered in the software is used to develop a stereotactic radiosurgery plan, and an intended three-dimensional dose distribution map is created to treat the lesion.
After the plan is developed, the box is loaded with multiple layers of film separated by spacers having tissue-equivalent characteristics. Thereafter, the three-dimensional radiation dose described by the stereotactic radiosurgery plan is delivered, while the hollow shell is supported by a head ring in a target-positioner box. Both the head ring and the target-positioner box are commonly used clinical components. Then, the multiple layers of film are removed from the box. If preferable to increase spatial resolution of the final measured dose distribution map, the box may be again loaded with multiple layers of unexposed film and the three-dimensional dose may be delivered thereto, with the angle of the box varied with respect to the hollow shell. These steps may be repeated a suitable number of times to obtain plural sets of multiple layers of film, all of which include the radiation dose recorded thereon. Thereafter, multiple dose images are obtained based on the multiple layers of film. Based on the multiple dose images, a measured three-dimensional dose distribution map is obtained. Thereafter, the measured three-dimensional dose distribution map is coregistered in the treatment-planning software with the intended three-dimensional dose distribution map that was previously developed, to ascertain any discrepancies therebetween to determine inaccuracies included in the original stereotactic radiosurgery plan.
In accordance with one aspect of the present invention, the box is supported by a rod, and the rod is slidably adjustable along its longitudinal axis to vary the position of the box along the axis within the hollow shell.
In accordance with another aspect of the present invention, the box is made of material opaque to visible light except for a diagonal strip that is made of material translucent to visible light. When the multiple layers of radiographic film loaded in the box thus constructed are exposed to radiation, indexing marks are recorded on the edges of the film, each film having a unique indexing mark. The indexing marks may be advantageously used to automatically orient and order the multiple layers of film.
The phantom system and method of the present invention record and verify a dose administered during stereotactic radiosurgery in three dimensions simultaneously. Because the system and method are designed to closely simulate the complete process of actual stereotactic radiosurgery performed on a patient, any spatial error included in each step of actual radiosurgery can be thoroughly encompassed in the simulation method of the present invention. The system and method, thus, allow for accurately adjusting a stereotactic radiosurgery plan based on the simulation result, prior to its actual application on a patient.